Typically in some cameras the film transport apparatus is motorized. In operation, a leading end portion of a filmstrip originally protruding or advanced from a film cartridge loaded into the camera is attached to a film takeup spool. The take-up spool is rotated after each exposure to advance successive frames of the filmstrip from the cartridge, across the focal plane of a taking lens, and onto the take-up spool. Each time the take-up spool is rotated, an unexposed frame of the filmstrip is positioned in the focal plane of the taking lens and an exposed frame is wound onto the take-up spool. When every one of the available frames have been exposed, a trailing end portion of the filmstrip remains attached to a film supply spool inside the cartridge. This end-of-film condition produces a sudden increase in the film tension and in the motor current as the take-up spool attempts to withdraw the remainder of the filmstrip from the cartridge. A film tension sensor responds to the sudden increase in the film tension or a current detecting sensor responds to the sudden increase in the motor current, by reversing the motor drive to rotate the supply spool inside the cartridge in place of rotating the take-up spool. The supply spool then draws the exposed frames of the take-up spool and winds them into the cartridge.
Rather than a film tension sensor or a current detecting sensor, a film perforation sensor may be provided for detecting a turn-around perforation in a trailing end portion of the filmstrip to reverse the motor drive to initiate winding the exposed frames into the cartridge. See commonly assigned U.S. Pat. No. 5,552,848 issued Sep. 3, 1996 and U.S. Pat. No. 5,508,770 issued Apr. 16, 1996.
In each design, simplicity is a goal in order to keep the manufacturing cost of the motorized camera relatively low.